The fabrication of semiconductor devices on semiconductor wafers requires that the semiconductor wafers be processed in a variety of different manners. Photolithography, chemical mechanical polishing (CMP), metal deposition, and so on, are all processes performed on semiconductor wafers in precise degrees to fabricate semiconductor devices. Usually, each of these different processes is performed using a different piece of semiconductor fabrication equipment. Therefore, a transport system is used to transport the semiconductor wafers among the different pieces of semiconductor fabrication equipment to achieve semiconductor device fabrication. Usually the semiconductor wafers are transported in pods, such as front-opening unified pods (FOUP's).
FIG. 1 shows a top view of a traditional pod transport system 100. The system 100 particularly includes a conveyer 104 on which the pods may be transported among various stations 102a, 102b, . . . , 102n. Each of these stations 102a, 102b, . . . , 102n may be a separate piece of semiconductor fabrication equipment, a storage place to store pods and their semiconductor wafers, and so on. Overhead transfer (OHT) units 106 are able to move the pods vertically.
One particular area 108 of the system 100 of FIG. 1 is shown in more detail in a side view in FIG. 2. One of the OHT units 106 has a pod 202a that it is vertically lowering onto one of the load ports 204. One of the robot arms 208 has lowered on its vertical axis 206 to pick up the pod 202b that has already been lowered onto one of the load ports 204. The other of the robot arms 208 has risen on its vertical axis 206 to deposit the pod 202c onto one of the conveyer belts 104. In this way, pods 202 are moved from the OHT units 106 to the load ports 204, and ultimately to the conveyer belts 104.
Unfortunately, the use of robot arms 208 that are vertically movable on the stationary vertical axes 206 is problematic. First, such usage compromises vertical clearance between the load ports 204 and the conveyors 104. This is disadvantageous, because vertical clearance of at least three meters is desirable so that new semiconductor fabrication equipment and tools can be easily moved in and moved out. Furthermore, the robot arms 208 and/or the stationary vertical axes 206 are typically located less than fifteen centimeters away from the OHT unit 106. This is also disadvantageous, because the OHT units 106 typically have proximity collision sensors that are triggered when anything is located less than fifteen centimeters away from them.
Therefore, there is a need to overcome these disadvantages. Specifically, there is a need for a pod transfer system that avoids comprising vertical clearance. There is also a need for such a transfer system that does not trigger the proximity collision sensors of the OHT units. For these and other reasons, there is a need for the present invention.